The present invention relates to a casing for centrifugal compressors.
The present invention also relates to a method for the manufacture of a casing for centrifugal compressors.
As is known, a centrifugal compressor is a machine into which a compressible fluid is introduced and is emitted with a pressure greater than its inlet pressure.
Centrifugal compressors are formed by a casing, or cylindrical body, which is closed at its ends and which contains the operating components of the machine.
Centrifugal compressors may comprise one or more stages and may be used for medium and/or high pressures, but in the remainder of the present description reference will be made in particular to multiple-stage centrifugal compressors.
Non-exhaustive examples of the possible uses of these centrifugal compressors are: reinjection of gas, recompression of gas, use of the compressor in association with gaseous-fuel feed plants in power generating machines, refineries, methanol, ammonia and urea synthesis plants and in high-pressure gas or liquid natural gas lines.
In order to understand better the technical problems involved in the present invention, reference should be made firstly to FIGS. 1 and 2 which show, respectively, a partially sectioned front view and side view of a casing for centrifugal compressors constructed in accordance with the prior art.
The casing for centrifugal compressors, according to the prior art, is indicated in its entirety in said FIGS. 1 and 2 by the reference number 10.
The casing for centrifugal compressors 10 has a substantially cylindrical body 11 which is at present obtained by means of forging (for example, using the materials ASTM A350 LF2, ASTM A350 LF3, ASTM A266 CL2).
The casing 10 has a cylindrical body 11 in which a plurality of openings 12 and 13 are formed by means of machining, the gas intake flange 14 and delivery flange 15 being positioned in said holes.
The operation of welding the nozzles of the intake flange 14 and delivery flange 15 is performed in the region of these openings 12 and 13.
In particular the nozzles of the flanges 14 and 15 are also obtained by means of forging and have a frustoconical body indicated by 16 and 17 respectively.
Each of these nozzles has a circular rim 18 and 19 at one end where the interface of the flanges 14 and 15 is performed by means of machining with machine tools.
The intake flange 14 and delivery flange 15 and the substantially cylindrical body 11 of the casing 10 are assembled together, by means of a welding process, for example electrode welding.
FIGS. 1 and 2 in fact show the welds 20 between the flanges 14 and 15 and the casing 10; the holes 24 and 25 used for raising the casing 10 are also shown.
In the known method, following this assembly operation, a post-welding stress-relieving heat treatment and all the checks envisaged by the production cycle are performed.
The last phase in this cycle consists of the operation for finishing the casing 10, the support feet 21, 22 and 23 and the intake flange 14 and delivery flange 15, said finishing generally being performed with machine tools.
The support feet 21, 22 and 23 are also fixed to the cylindrical body 11 of the casing 10 by means of welds 29.
It should be noted, however, that the casings for centrifugal compressors constructed in accordance with the prior art have certain major drawbacks due precisely to the abovementioned way in which the known casings are currently made.
Firstly, it should be pointed out that today it is necessary to perform assemblyxe2x80x94by means of the welding processxe2x80x94of the casing 10 with the intake and delivery flanges 14 and 15 as well as the support feet 21, 22 and 23 and centring blocks.
Moreover, it necessary to perform at least one checking and stress-relieving heat treatment operation on the welded joints.
These facts mean that the method of the prior art, although it has proved to be fairly effective over the years, is at the present time excessively uneconomic both in terms of the manufacturing time and in terms of the costs for production of the centrifugal compressors.
The object of the present invention, therefore, is to provide a casing for centrifugal compressors which may be prepared in a simple, rapid and efficient manner.
Another object of the invention is to provide a casing for centrifugal compressors which does not require time-consuming checking operations afterwards.
Another object of the invention is to provide a casing for centrifugal compressors where it is possible to achieve a substantial reduction in the delivery time of the finished casing as well as a substantial reduction in the price of the finished casing.
A further object of the invention is to provide a casing for centrifugal compressors which can be produced in a simple and low-cost manner, without the need to use costly materials or complex technology.
These and other objects are achieved by a casing for centrifugal compressors, comprising a substantially cylindrical body which is internally hollow and has its own central axis, characterized in that the internal cavity of the abovementioned casing has an internal central axis substantially coinciding with the axis of rotation of the machine and offset with respect to the abovementioned central axis of the external surface of the abovementioned cylindrical body so that the abovementioned casing has an eccentric shape, having portions of different thickness, so as to create zones where the attachment point for the gas inlet counterflange and outlet counterflange can be formed.
In particular, the central axis is offset, in a vertical direction, with respect to the central axis of the external surface of the cylindrical body of the casing.
According to a preferred embodiment of the present invention, the casing for centrifugal compressors has an attachment point for the gas inlet and outlet flanges, which is directly obtained by means of machining with machine tools, on the side with a greater thickness of the cylindrical body of the external surface of the casing.
According to another preferred embodiment of the present invention, the casing for centrifugal compressors has a pair of flat surfaces, formed by means of a machine tool, in the region of the intake and delivery ducts.
According to yet another preferred embodiment of the present invention, the holes for raising the said casing and the threaded holes for connection to the support feet of the casing are formed on the substantially cylindrical body of the casing for centrifugal compressors.
The present invention also relates to a method for the manufacture of a casing for centrifugal compressors which comprises a step for formation of a cavity inside the casing for compressors, where the abovementioned cavity has an internal central axis which is offset, in a vertical direction, with respect to the central axis of the external surface of the cylindrical body of the casing, and a step involving machining with machine tools in the zone of greater thickness of material of the abovementioned cylindrical body of the casing, in order to form the attachment point for the abovementioned gas inlet and outlet counterflanges.
The method for the manufacture of a casing for centrifugal compressors also envisages the fact that a pair of flat surfaces are formed in the region of the intake and delivery ducts by means of a machine tool.